Major Hurricane Willa is just 36 hours away from landfall along Mexico’s west-central coast on Tuesday evening. The dangerous Category 4 Hurricane was a meager Tropical Storm just two days ago before undergoing an unexpected Rapid Intensification cycle (RI), leaving residents with little warning to prepare for its devastating impacts. After crippling the Mexican states Sinaloa and Nayrit Tuesday night, Willa’s remains will continue northeastward towards the Texas coast. While dousing southern Texas with heavy downpours, Willa’s remnants will interact with a frontal boundary, prompting re-intensification into a coastal low that could become the first Nor’Easter of the cold season.

Early Monday afternoon, Willa intensified into a Category 5 hurricane with maximum sustained wind speeds of 160 mph. Eye-wall replacement prompted a slight decrease in Willa’s winds Monday afternoon, prompting weakening to 155 mph as of 3pm CDT, which is Category 4 strength. Willa still looks impressive on satellite despite clouds covering the eye, a consequence of eye-wall replacement. The hurricane is moving north at 8 mph, and is expected to curve northeastward toward western Mexico Tuesday morning.

The magnitude of Willa’s intensity is quite astonishing considering that the hurricane was a tropical storm with maximum sustained winds of just 40 mph only two days before this writing. Widely used American global and regional hurricane guidance only projected Willa’s intensity to be between Category 2 to weak Category 4 intensity as recently as Sunday morning. The following graphic from NOAA’s Environmental Modeling Center (EMC) illustrates the dichotomy between the forecast and reality. Only the official NHC forecast and the Navy’s hurricane model (CTCX) projected Willa to slowly reach marginal Category 4 intensity. Instead, as the “best” line indicates, Willa rapidly intensified into a powerful Category 5 hurricane earlier this afternoon.

The associated errors in model guidance are even more eyebrow-raising. The following plot depicts aggregated model absolute errors in wind speed at 12 hourly lead-time intervals. The errors are remarkably large. The GFS in particular was up to 55 kt (63 mph) too low only four days before this writing, and just five days before landfall.

Willa is the third hurricane to impact North America this season that has underwent rapid intensification just before landfall. Florence and Michael were also major hurricanes that underwent the same rapid intensification. Notably, Michael unexpectedly continued to intensify until making landfall.

The National Hurricane Center defines rapid intensification as “an increase in the maximum sustained winds of a tropical cyclone of at least 30 kt in a 24-h period.” Rapid deepening of pressure was formerly used to denote a dramatic intensification of a tropical cyclone, but winds have since been recognized as more relevant to impacts. So we know that Willa went through RI, but that still leaves a question hanging. What conditions in particular caused Willa to explode into a Category 5 hurricane Monday afternoon?

Sea Surface Temperatures (SSTs) of at least 26°C (78.8°F) are generally required for tropical cyclone intensification. As the above SST analysis from Sunday demonstrates, SSTs have been 28°C – 30°C along Mexico’s western coast, much warmer than the 26°C threshold. But it is not enough for a tropical system to merely be exposed to warm sea surface temperatures. The warm ocean temperatures must extend sufficiently deep below the surface, throughout the oceanic mixed layer. If the warmth only encompasses the sea surface, waves will quickly churn out the warm water, draining the tropical cyclone of potential energy. In the atmosphere, wind shear should be low so that the cyclone’s circulation is not disrupted. Further, since hurricanes are fueled through condensation, there ought not to be dry air-intrusion which can limit the release of latent heat. In Willa’s case, all prove true.

Unlike Michael, Willa is not expected to continue intensification just before landfall. Eye-wall replacement is expected to continue until Willa encounters a region of southwesterly wind shear Tuesday afternoon. These factors will contribute to gradual weakening until landfall as a life-threatening strong Category 3 or weak Category 4 hurricane. The storm surge will be capable of swallowing entire villages along the coasts of Sinaloa and Nayrit. Rainfall of up to 18″ will drown communities in the storm’s path, especially those along western mountain slopes. Roaring winds in excess of 100 mph will be enough to rip the roofs and sides from buildings and topple trees and power-lines. The infrastructure in many of the areas to be impacted by Willa are not very robust. Some communities could be cutoff from the outside world for over a week.

Willa will quickly dissipate after being torn apart by central Mexico’s mountainous topography. The storm’s weak remnants will reach the Texas coast and interact with another weak low-pressure system by Wednesday, inundating the state with more heavy rain. 4-8″ of rain has already fallen in central and southern Texas throughout the last week, notably prompting a bridge collapse near Austin. The undead version of Willa is expected to dump an additional 2-5″ of rain into areas where water levels have yet to fully subside. That’s not good at all.

Vorticity associated with Willa’s remnants will overlap a pre-existing surface low along a weak frontal boundary. Vorticity is localized spinning motion in the air and its transport can stretch the atmosphere, resulting in rising motion. Intensification of the newly combined low pressure system will thereafter occur along the Gulf and Southeast coasts through late weak. The storm will follow the temperature gradient between warm tropical air to the south and cold Canadian air drawn into the Southeast by an intensifying shortwave trough.

As discussed in last night’s Sunday Storm, the intensifying extra-tropical cyclone that will partially result from Willa appears likely to continue up the East Coast, becoming the first nor’easter of the cool season. The evolution of the potential coastal storm depends heavily on features that have yet to form but will impact the future position and intensity of the jet stream. These factors include the rate of decay of Willa, the strength and timing of the aforementioned shortwave trough, and the intensity of the antecedent cold air mass in the Northeast. We will continue to provide updates on the potential nor’easter as confidence increases throughout the week.

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.